A great variety of small portable devices such as personal digital assistants (PDA), multi-function cell phones, digital cameras, music players, etc. have become widely available. These devices use a central processing unit (CPU) or microcontroller and a mass-storage memory such as a hard drive or flash memory. These small devices are often cost and size sensitive.
Hard disks and other mass storage devices are being replaced or supplemented with solid-state mass storage such as flash memories. Flash memories use non-volatile memory cells such as electrically-erasable programmable read-only memory, (EEPROM), but are not randomly accessible at the byte level. Instead, whole pages or sectors of 512 bytes or more are read or written together as a single page. NAND flash memory is commonly used for data storage of blocks. Pages in the same block may have to be erased together, and limitations on writing may exist, such as only being allowed to write each page once between erases.
These small portable electronic devices often are able to connect to a host computer such as a personal computer (PC). While a proprietary connector may be used, a connector for a standard expansion bus is preferable. Universal-Serial Bus (USB) is often used to connect such portable flash-memory devices to a PC.
USB uses one pair of differential lines that are time-duplexed, or used for transmission in both directions, but at different times. This may limit performance when data needs to be sent in both directions at the same time. The current USB 2.0 standard provides that the host, such as the PC, controls the bus as the bus master, while USB devices plugged into the host act as slave devices. A USB controller on the host PC generates data transfer transactions and waits for USB devices to respond, either by transmitting requested data to the host, or by writing host data into the USB device's memory.
Since memory on a USB device may be busy or slow, sometimes the host's request cannot be processed immediately. The host may send the request, then periodically poll the USB device to see whether the data is ready. Also, when the host is idle, the host may need to periodically poll the USB device to see if the USB device needs to transfer information to the host. This periodic polling may be used for other purposes as well, such as for polling a mouse for movement.
While polling is useful, since it allows the host to completely control the USB bus, power is consumed each time a packet is sent for polling. While this power is small, for low-power or battery-powered devices, the amount of power consumed may be significant and undesirable. Also, the USB device or host may otherwise be in a low-power sleep or suspend state, and have to wake up into a higher-power state to perform or respond to the polling. There may be significant time and energy required to wake up from the suspend or sleep state, and then to re-enter the suspend or sleep state once polling is done.
What is desired is a USB device and USB host that have lower power. A USB system that does not require polling is desirable. Bus protocols and transactions that avoid polling are desirable to be applied to USB to reduce energy consumed by polling.